The following Patent Document 1 discloses a TOF (Time Of Flight) type distance image measurement device. This distance image measurement device repeatedly emits a probe light having a predetermined pulse width toward a subject and measures a period of time from an emission time to a return time of the probe light, that is, a time of flight of the probe light to thereby measure a three-dimensional distance image up to the subject. In this device, a phase difference between pulses at an emission and pulses at a return of a probe light is measured as a time of flight.
Time of flight measuring methods include one that determines a ratio of the amount of charge accumulated in a plurality of accumulation regions formed in each pixel. In Patent Document 2, the accumulation timings in the charge accumulation regions are differentiated. The ratio of the amount of charge generated in each accumulation region due to incidence of one reflected pulse light is proportional to the time of flight, by eliminating a background light component. For example, an accumulation timing in one accumulation region is set in accordance with the period of time from a rising time to a falling time of probe light pulses, and an accumulation timing in the other accumulation region is set in accordance with the period of time from a falling time and a rising time. In this case, with zero time of flight, if the amount of accumulated charge in one accumulation region is 100%, the amount of accumulated charge in the other accumulation region is 0%, so that the distance up to a target is zero. The longer the time of flight, the more the ratio of the amount of accumulated charge in the other accumulation region increases, and thus the distance up to a target is determined according to the ratio of the charge amount.    Patent Document 1: U.S. Pat. No. 6,373,557 specification    Patent Document 2: International Publication No. WO 2006/010284 pamphlet